The invention relates to a metering apparatus for liquids, particularly for use in clinical chemistry, comprising a motor-driven piston/cylinder system; an optical position-determining system whose transparent pulse scale, provided with nontransparent marks, is affixed to the piston; an optical/digital measuring circuit which converts the stroke of the piston into a corresponding number of digital pulses; and a control, computer and comparator circuit which incorporates a counter counting the digital pulses and which controls the stroke of the piston on the basis of the preselected volume to be metered.
An apparatus of this type is known from U.S. Pat. No. 3,796,239. That reference discloses a desk set which must be connected to the electric line. The piston/cylinder system is driven by an AC-powered reversible motor having two windings, one for the forward and one for the reverse direction. Attached to the piston is a transparent slide-rule scale having four marks which correspond to the volumes of 25, 50, 100 and 200 microliters. These marks are optically scanned. For this purpose, the transparent slide-rule scale moves between a light transmitter and a light receiver in the form of a differential photodiode. The information identifying the one of the four possible volumes that is to be dispensed is entered into the desk set as a program by means of a punched card. As soon as the programmed mark is recognized by the photocells, the current supply to the motor is shut off, and the friction of the transmission and of the piston is relied on to brake the momentum of the motor.
A further metering apparatus is known from U.S. Pat. No. 3,756,456. In that apparatus, the motion of the piston is transmitted to a disk which is provided with a regular line grid. Light is projected onto that disk through a scanning plate which is likewise provided with a regular line grid. Now while an optical position-determining system of this type is more accurate in principle than the one to which U.S. Pat. No. 3,796,239 relates, its accuracy is reduced by the slight slip which occurs between the disk and the piston.
As is explained in detail in U.S. Pat. No. 3,756,450, accuracy problems also arise from the fact that liquid to be metered keeps flowing even after the drive motor has been turned off. Allowance is made for this afterflow of liquid in that the counter in which the pulses coming from the rotating coding disk are registered in preset for a given number of pulses before the start of the metering operation. Since it is very difficult to determine the number to be so preset, the use of this expedient in everyday routine laboratory practice is out of the question. Besides, it increases the complexity of the required electronic circuitry since a counter and a counter-position comparator are additionally needed.
In any laboratory where serial analyses are performed, the various sequences of operations, and particularly the many metering operations, must be continually rendered more efficient. Now fully automatic analyzers call for high capital outlays, full utilization of their capacity, and specially trained personnel. Such equipment is of great complexity, and any malfunction entails expensive down time. This is why electronic metering apparatuses have been developed wherein the pistons of the metering syringes are moved either by means of electronically controlled stepping motors or, as described above, through alternating-current motors controlled by optical measuring systems. However, the size and weight of the motors and of the mechanical power-transmission system are such that these apparatuses must be constructed as stationary installations. This militates against ease of operation; and, in addition, such apparatus requires a continuous line-power supply. The size and weight of the apparatus are also reflected in its overall cost.